1. Field of the Invention
The present invention relates generally to the field of energy conversion for lighting and more specifically to an electronic ballast suitable for use with gaseous-discharge lamps.
2. Description of the Prior Art
Gaseous-discharge lamps, lamps in which light is generated when an electric current, or discharge, is passed through a gaseous medium, are not new to the lighting field. Fluorescent-type gaseous-discharge lamps have been employed for years to provide relatively efficient indoor lighting such as for office buildings. Recently, sodium-vapor-type gaseous-discharge lamps have been employed to replace less efficient lamps in outdoor lighting applications. For example, 250 watt sodium-vapor lamps are commonly used in street lights to replace 400 watt mercury-vapor lamps which are less efficient and which generate less light output. Sodium-vapor lamps in 70, 100, 400, and even 1000 watt sizes are also commonly used.
Unlike incandescent lamps which are self-limiting as a result of their positive-resistance characteristic, gaseous-discharge lamps have a negative-resistance characteristic. For this reason, gaseous-discharge lamps are operated in conjunction with a ballast which provides the requisite current limiting. Traditionally, ballasts are of core and coil construction. One form is that of a simple choke which provides an inductive impedance for current limiting. Another form is that of a transformer. The transformer form permits voltage conditioning such as providing a high break-down potential which is required for starting instant-start-type fluorescent lamps by ionizing to a plasma the gas therein. For rapid-start-type fluorescent lamps, a pair of windings are included in the transformer for energizing the lamp filaments and, separating the filament windings, a high-voltage winding having a high reactance for current limiting. Alternatively, a magnetic shunt may be included in the transformer to limit the energy transferred through the magnetic path.
Unfortunately, traditional core-and-coil-type ballasts are relatively inefficient having substantial heat generating losses that are generally equally divided between copper losses in the coil and core losses in the relatively inexpensive grades of iron employed therein. For example, it is not unusual for a tranditional core-and-coil-type ballast employed in a dual 40 watt lamp fixture to dissipate from 15 to 20 watts causing the ballast to run quite hot. Further, in many applications, such as in office buildings, this ballast-generated heat must be removed by air conditioning equipment which is itself relatively inefficient. Another problem is that core-and-coil-type ballasts are relatively heavy requiring that associated fixtures be more substantial than would otherwise be necessary.
The regulation afforded by traditional core-and-coil-type ballasts is also relatively poor. Typically, the operating level of fluorescent fixtures employing such ballasts varies as the square of the power-line voltage. Thus, in many applications, excessive lighting, dissipating excessive power, is often employed to insure that minimum lighting levels are achieved.
Compensation, at least in part of variations in line voltage, is often afforded for sodium-vapor-type gaseous-discharge lamps by employing therewith so-called constant-voltage, or ferro-resonant, type transformers having inherent current limiting. Unfortunately, such transformers are relatively expensive, heavy and bulky. Sodium-vapor lamps, moreover, present another regulation problem. Unlike fluorescent lamps across which a voltage drop is developed that remains relatively constant with lamp life, the voltage drop developed across a sodium-vapor lamp often varies as much as two to one during the life of the lamp. As a result, to insure that minimum light levels are achieved, sodium-vapor lamps are often overdriven during most of their lives, at the expense of both power and life, and/or excessive lighting is employed.
Among other problems associated with gaseous-discharge lamps is that they are less efficient when operated at the normal 60 Hz line frequency than when they are operated at higher frequencies. Sodium-vapor lamps often require special starting circuitry. Fluorescent lamps are often difficult to start when cold and, as a result, flicker for some time. Fluorescent lamps require core-and-coil-ballast phasing both to reduce stroboscopic effects and to increase the power factor such lamps present to the line via the ballast.
What may be referred to as an electronic ballast is disclosed in U.S. Pat. No. 4,277,728 which issued to C. Stevens. Included is a switching power supply for developing a source of DC power from AC line power, an inverter for developing a source of high frequency AC power from a portion of the DC power and an RF-type resonant network for coupling a portion of the high frequency AC power to a gaseous-discharge lamp. The resonant network both limits the lamp current and provides a voltage step-up for starting the lamp.
By increasing the frequency of power used to drive a gaseous discharge lamp, the electronic ballast disclosed by C. Stevens is advantageous in that it permits the lamp to operate more efficienlty. It is also advantageous in that the increased frequency permits much smaller, lighter and more efficient components to be employed for the current limiting resonant network.
In an embodiment illustrated in FIG. 4B of the C. Stevens disclosure, the inverter is shown to include an oscillator and drivers driving a pair of transistors. The transistors operate as switches series connected across the output of a DC power supply in what may be referred to as a totem-pole configuration. The resonant network is shown to include a pair of inductors and a capacitor connected in a T-type configuration. More specifically, the two inductors are connected in series from the juncture of the switching transistors to one end of the lamp, the distal end of which is connected to a common potential. The capacitor is connected from the juncture of the inductors to the common potential. The common potential is developed at the juncture of a pair of capacitors series connected in a voltage-divider configuration across the output of the DC power supply. A phase detector is also included for synchronizing the frequency of the oscillator with the resonant frequency of the T-type network. Further, a current-sensing resistor is included, evidently for developing a signal for controlling the power supply.
By synchronizing the frequency of the oscillator with the resonant frequency of the network, the transistors switch at current-null points, switching losses are reduced, the transistors are protected and the switching transistors appear to be driving a resistive load. Further, synchronization insures that a maximum voltage step up will occur.
The electronic ballast disclosed by C. Stevens does not employ a simple power supply of the type which includes a bridge, or other form of rectifier, to develop pulses of direct current from the AC power line and a filter capacitor directly connected to the rectifier to develop a relatively constant voltage from the pulses of direct current. Such a simple power supply is disadvantageous in that all the current is drawn from the AC power line in synchronization with the peaks thereof. these current peaks cause power factor problems and problems sometimes referred to as third harmonic distortion problems. The occasional use of such simple power supplies causes little problem. However, where a large number of such supplies are connected to a single power line, such as to provide power for the lighting in a whole office building, problems with the pole transformer and power line wiring may result.
To avoid these problems, the electronic ballast disclosed by C. Stevens employs a relatively complex power supply having a switching regulator disposed between the bridge rectifier and the filter capacitor. The switching regulator interconnects the rectifier and the filter capacitor for brief periods at a high, 20 kHz, rate to form a train of current pulses at the 20 kHz rate. The regulator includes circuitry for varying the width of the current pulses in synchronization with the power-line frequency to develop wide pulses during the peaks of the power-line cycle and narrow pulses during the valleys thereof. As a result, the power supply disclosed by C. Stevens draws power from the AC line so as to appear as a load having a near unity power factor. Unfortunately, the power supply disclosed by C. Stevens is relatively complex and expensive.
Other disclosures which may be considered of interest include the U.S. Pat. Nos. 4,060,751, 4,127,798, 4,251,752 and 4,253,046 issued to T. Anderson, J. Anderson, J. Stolz and Gerhard et al, respectively.